Great idea Lilly! I never thought of a post like this so thanks!
Do insure high priced or big ticket items - Sadly, some items/boxes don't always quite make it to there destination either due to nosy people or due to mail getting lost.
Do send as much support as you can - It is the highlight of many of their days when they get a package or a letter of support from a complete and caring "stranger". Letters have just as much impact as packages so don't think you aren't helping.
Don't send homemade goods - Those will be thrown away immediately for their safety. I am sure most of us know this.
I'll update if I remember anymore things.
Don't send pork or any products containing it - This may not necessarily apply to everyone so check but I know it is not allowed in Iraq.